Method for forming distributed Bragg reflectors in optical media

ABSTRACT

A device for transmitting radiation is provided. The device includes a glass optical waveguide having a core. A Bragg grating is at least partially formed within the core. Associated with the Bragg grating is a transmission spectrum that includes a band in which incident radiation is attenuated, the peak attenuation being at a wavelength λ. The transmission spectrum of the Bragg grating is such that the full width at half maximum measured in transmission is greater than or equal to (1.5 nm/1558.5 nm) λ and peak attenuation within the band is greater than or equal to 90% of full scale transmission. Attenuation exceeds 50% of the peak attenuation everywhere within the full width at half maximum of the transmission spectrum.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of application Ser. No. 08/199,366,filed on Feb. 18, 1994, now abandoned, which application is in turn aContinuation-In-Part of application Ser. No. 07/995,726 filed Dec. 23,1992 and issue as U.S. Pat. No. 5,309,260 on May 5, 1994, which areincorporated herein by reference.

FIELD OF THE INVENTION

This invention pertains to the processing of optical waveguidingarticles such as optical fibers, and more specifically to the formationof passive optical components that are integrated with such waveguidingarticles, by using actinic radiation to modulate the refractive index.

ART BACKGROUND

Certain optical media, including at least some silica-based opticalfibers, can be modified by exposure to electromagnetic radiation in anappropriate spectral range. (Such radiation, typically ultravioletradiation, is referred to below as "actinic" radiation.) That is,exposure of a photosensitive optical fiber (or other optical medium) toactinic radiation may cause the refractive index to change in theexposed portion of the medium. A periodic pattern can be imposed on theimpinging radiation by, e.g., superimposing a pair of beams ofsubstantially monochromatic radiation from, e.g., a laser, to create aninterference pattern. When such a patterned radiation field impinges onan optical fiber or other optical waveguide having a core of theappropriate photosensitivity, a corresponding pattern is imposed on thecore in the form of periodic (or quasiperiodic) fluctuations in the corerefractive index. Such a pattern, which is often referred to as a "Bragggrating" or a "distributed Bragg reflector (DBR)" can behave as aspectrally selective reflector for electromagnetic radiation. Bragggratings formed in this manner are particularly useful as end-reflectorsin optical fiber lasers. These Bragg gratings are useful both becausethey are spectrally selective, and because they are readily incorporatedin the same optical fiber that supports the active laser medium.

A technique for creating these Bragg gratings is described in U.S. Pat.No. 4,725,110, issued to W. H. Glenn, et al. on Feb. 16, 1988, and U.S.Pat. No. 4,807,950, issued to W. H. Glenn, et al. on Feb. 28, 1989. Anoptical fiber laser having a DBR-terminated cavity is described in G. A.Ball and W. W. Morey, "Continuously tunable single-mode erbium fiberlaser", Optics Lea. 17 (1992) 420-422.

Bragg gratings are useful as passive optical components for otherapplications besides end-reflectors in fiber lasers. For example, Bragggratings are useful as spectral filters for wavelength-divisionmultiplexing and other optical signal-processing applications. Anoptical filter which comprises a Bragg grating formed in an opticalfiber is described in U.S. Pat. No. 5,007,705, issued to W. W. Morey, etal. on Apr. 16, 1991.

We have observed that when a pair of intersecting laser beams is used toform a Bragg grating in an optical fiber, the resulting grating mayexhibit certain optical properties that are generally undesirable.Specifically, the reflectivity spectrum of the grating may exhibit oneor two relatively sharp subsidiary peaks, or a regularly spaced sequenceof such peaks, to one side of the central peak, generally theshort-wavelength side. (These subsidiary peaks are hereafter referred toas "fine structure".) This fine structure is undesirable, for example,in a feedback stabilization system in which the output wavelength of alaser is locked onto the central peak of a Bragg grating. If the gratinghas subsidiary peaks, it is possible for the tuning of the laser toshift to a subsidiary peak in response to an environmental disturbance.Thus, the presence of fine structure may make a laser system of thiskind less robust against environmental disturbances.

By way of illustration, FIG. 1 shows an experimentally measuredtransmissivity spectrum of a typical Bragg grating formed in an opticalfiber. (In the absence of loss, the sum of transmissivity andreflectivity is 100%.) The spectrum includes a broad main peak 10 and aseries of subsidiary peaks 15.

We attribute this fine sideband structure to interference effectsrelated to the average axial profile of the refractive index in thegrating region. (By the "axial" direction is meant the propagationdirection of electromagnetic radiation in the grating.) That is, therefractive index of the fiber (or other waveguiding medium) in thegrating region is conveniently described in terms of a perturbation δ(z)which represents the difference between this index and the refractiveindex of the unexposed fiber, and in terms of the variation of theperturbation along the axial direction (i.e., the z-direction). Theperturbation varies periodically, in step with the successive light anddark fringes in the interference pattern that created it. However, eachof the interfering beams has a spatially varying intensity profile inthe plane perpendicular to the propagation direction of the beam. Thisprofile is typically Gaussian in shape. The intensity profiles of theinterfering beams define the spatial extent of the Bragg grating, andmodulate the amplitude of the periodic refractive index perturbation. Asa result, the perturbation δ(z) generally takes the form of a periodicseries of peaks enclosed by an envelope, typically Gaussian in shape,which is maximal at or near the center of the grating, and falls off tozero at the edges of the grating. If the perturbation is averaged overan axial distance much larger than the grating period, e.g., over ten ormore periods, then the resulting average perturbation will of coursehave the same shape as this envelope.

The existence of such envelopes is well known. In fact, it is well knownthat an envelope having, e.g., a rectangular shape will give rise toside lobes in the resulting reflectivity spectrum. (See, e.g., H.Kogelnik, "Filter Response of Nonuniform Almost-Periodic Structures",The Bell System Technical Journal 55 (1976) 109-126.) However, the basicreason for this effect is that the grating has a limited spatial extent.By contrast, the fine structure discussed above is a consequence of thespatially averaged perturbation. This average perturbation is effectivein certain respects as a "background" perturbation, which has physicalsignificance separate from that of the rapid modulations (i.e., the"lines") of the grating. Until now, a full discussion of the effects ofthe average perturbation on spectral structure has not appeared in therelevant technical literature. In particular, practitioners in the anhave hitherto failed to address possible techniques for mitigating (orfor enhancing) the resulting fine structure.

It will be noted that fine structure will ordinarily not be resolved ona conventional optical spectrum analyzer. Such analyzers typically havea resolution, at best, on the order of 1 angstrom. Thus, conventionallymeasured spectra often fail to illustrate fine structure which isnonetheless present in the waveguide.

In order to accurately resolve fine structure, it is often necessary toemploy a high resolution measurement technique such as a tunableexternal cavity semiconductor laser and power detector which can bestepped in wavelength increments significantly smaller than 1 angstrom,on the order of tenths of angstroms or less. Such a technique can beperformed with an HP external cavity tunable laser source model #8168A.

SUMMARY OF THE INVENTION

We have succeeded in demonstrating, by computer simulations, that if theprofile of the average refractive index perturbation δ_(ave) (z) isapproximately pulse shaped in some portion of the Bragg grating, thegrating may behave as a resonant cavity with respect to certainwavelengths of light (The term "light" is used herein to refer toradiation in the ultraviolet, visible, and infrared regions of theelectromagnetic spectrum.) Interference within this cavity, akin toFabry-Perot resonance, can account for the sidebands, or fine structure,that we have observed in laboratory experiments. We have furthermorefound a method for mitigating or enhancing this fine structure.

Accordingly, the invention in one embodiment involves a method forforming a Bragg grating (also referred to as a distributed Braggreflector, or "DBR") in a photosensitive optical medium. The DBR willexhibit a reflectivity spectrum that includes a main peak which has ameasurable amplitude. The DBR comprises a material region of the body inwhich the refractive index is the sum of an initial refractive index andat least a first, spatiaily periodic or quasiperiodic, refractive indexperturbation. (Art example of a quasiperiodic grating is a grating whoseperiod varies linearly with position, i.e., a so-called "linearlychirped" grating.) This sum has at least one vacuum Bragg wavelength,i.e., a vacuum wavelength of electromagnetic radiation that satisfiesthe Bragg condition in at least a portion of the DBR and as a result isrelatively strongly reflected by the DBR.

The method includes the step of producing two non-collinear beams ofelectromagnetic radiation having an actinic wavelength. Each of thesebeams will typically have, in cross section, a Gaussian intensityprofile. More generally, each beam will have an intensity profile thatincludes, along the beam diameter, at least one rising portion and atleast one falling portion. The two beams are impinged on at least aportion of the medium, such that an interference pattern is created onthe impinged portion, resulting in the first perturbation in theimpinged portion.

As a result of this actinic exposure, the impinged portion acquires abackground refractive index that is defined by summing the initial indexand the first perturbation and spatially averaging the sum over at leastten DBR periods or quasiperiods. In general, this background index willvary with position along the impinged portion such that it has risingand falling portions corresponding to the rising and falling portions ofthe beam intensity profiles. At any point within the impinged portion,the vacuum Bragg wavelength will generally be dependent on thebackground index at that point.

The impinged portion will generally include at least one segment, to bereferred to as a "resonant segment", that has first and second ends suchthat the background index is rising at the first end and falling at thesecond end. Each resonant segment has the further property that, in theabsence of any refractive index perturbation other than the firstperturbation described above, there is at least one vacuum wavelength ofelectromagnetic radiation that satisfies the Bragg condition at thefirst and second ends, but fails the Bragg condition in a regionintermediate the first and second ends. The resonant segments willgenerally contribute subsidiary peaks to the reflectivity spectrum ofthe DBR.

In contrast to methods of the prior art, the inventive method in oneembodiment further comprises, before or after the impinging step, thestep of causing a second refractive index perturbation in at least partof the impinged portion, such that the subsidiary peaks are modified. Inalternate embodiments of the invention, the subsidiary peaks aresuppressed or enhanced.

An alternate embodiment of the invention involves a step that isalternative to, or in addition to, the second perturbing step. Thisalternate embodiment includes, during the impinging step, the step ofvarying the period of the perturbation with axial position, such thatthe subsidiary peaks are suppressed or enhanced.

In a further aspect of the present invention, a device for transmittingradiation is formed. The device includes a glass optical waveguidehaving a core. A Bragg grating is at least partially formed within thecore. Associated with the Bragg grating is a transmission spectrum thatincludes a band in which incident radiation is attenuated, the peakattenuation being at a wavelength λ.

The transmission spectrum of the Bragg grating is such that the fullwidth at half maximum measured in transmission is greater than or equalto (1.5 nm/1558.5 nm) λ and peak attenuation within the band is greaterthan or equal to 90%. Attenuation exceeds 50% of the peak attenuationeverywhere within the full width at half maximum of the transmissionspectrum.

In another aspect, the present invention provides a device fortransmitting radiation. The device includes a glass optical waveguidehaving a core with a Bragg grating formed in the waveguide. Thereflectivity spectrum of the Bragg grating includes a band in whichincident radiation is reflected. Reflection within the band is greaterthan 90% over at least 2 contiguous nanometers.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a transmissivity spectrum, generated by computationalsimulation, of a typical Bragg grating formed in an optical fiber.

FIG. 2 is a schematic diagram showing the optical arrangement of anexemplary interferometer of the prior art that is useful in practicingthe inventive method.

FIG. 3 is a simplified diagram illustrating the spatial variation ofaverage refractive index in a typical Bragg grating formed in an opticalmedium.

FIG. 4 is a simplified diagram illustrating the mechanism by whichinterference effects may arise in a Bragg grating having a pulse-shapedaverage refractive index profile.

FIG. 5 is a simplified diagram further illustrating the mechanism bywhich interference effects may arise in a Bragg grating having apulse-shaped average refractive index profile.

FIG. 6 is a refractive index profile, generated by computationalsimulation, of the Bragg grating of FIG. 1. The average refractive indexperturbation is expressed, in this figure as well as in FIGS. 7 and 8,in terms of vacuum wavelength. Also shown are the upper and lower boundson the vacuum wavelength for which the grating, at a given point, isreflective.

FIG. 7 is a profile, generated by computational simulation, of therefractive index perturbation produced by an exemplary index-modifyingexposure, according to the invention in one embodiment.

FIG. 8 is the refractive index profile, generated by computationalsimulation, of the Bragg grating of FIG. 6, after being subjected to theindex-modifying exposure of FIG. 7.

FIG. 9 is the transmissivity spectrum, generated by computationalsimulation, of the modified Bragg grating of FIG. 8.

FIG. 10 is a simplified diagram illustrating the modification of theinterference effects of FIG. 5 such that the resulting sidebands areinverted relative to the central reflectivity peak. According to theinvention in one embodiment, this modification is achieved by modulatingthe average refractive index.

FIG. 11 is a transmissivity spectrum, generated by computationalsimulation, of the Bragg grating of FIG. 6 after an index-modifyingexposure for enhancing the fine structure, according to the invention inone embodiment.

FIG. 12 is a reflectivity spectrum of a hypothetical Bragg grating,illustrating the tendency of some subsidiary peaks to broaden the mainpeak, and the tendency of other subsidiary peaks to add roughness to theside of the main peak.

FIG. 13 is a transmissivity spectrum of a Bragg grating having a fullwidth at half maximum on the order of 8 nm.

FIG. 14 is a schematic representation of a reflectivity spectrum of theBragg grating of FIG. 13.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

We have found it advantageous to cream the interference pattern using ascanning interferometer of a design in which the translation of asingle, translatable mirror can shift the position of the interferencepattern along the fiber while preserving its registration. As aconsequence, the fiber can optionally be kept stationary betweenrespective exposure steps leading to the formation of multiple Bragggratings. An exemplary such interferometer is described, e.g., in U.S.Pat. No. 4,093,338, issued to G. C. Bjorklund, et al. on Jun. 6, 1978.The optical arrangement of the exemplary interferometer is illustratedin FIG. 2. Such an optical arrangement includes laser source 80,translatable mirror 85, rotatable mirror 120, and mirrors 90, 100, and110. The interfering beams converge on photosensitive optical medium130, which is exemplarily an optical fiber. The interference pattern canbe shifted (without affecting its phase) along the fiber by translatingmirror 85. In general, the periodicity of an interference pattern can bechanged by adjusting the angle of intersection φ of the interferingbeams. In the exemplary interferometer, this is achieved, withoutchanging the path-length difference between the interfering beams, byrotating mirror 120. A sequence of two or more gratings is readilyformed on a single optical fiber by translating mirror 85 and thenexposing a new portion of the fiber.

According to a preferred method for making the Bragg gratings, the fiberis first clamped into position to assure that the regions to be exposedare straight. The fiber is subjected to an effective exposure ofradiation, typically ultraviolet light. Various appropriate sources ofultraviolet light are available and known to those skilled in the art.

By way of illustration, we have found that an excimer-pumped, frequencydoubled, tunable dye laser emitting at about 245 nm is an appropriateexposure source. We have described the use of such an exposure source incopending U.S. patent application Ser. No. 07/878791, filed on May 5,1992 by D. J. DiGiovanni, et at., which we hereby incorporate byreference. As discussed therein, this exposure source is useful formaking gratings in highly erbium-doped, silica-based optical fibers.These fibers are typically exposed to 2-mJ pulses at a repetition rateof 20 pulses per second. A cylindrical lens focuses the laser light intoa band about 0.5 cm long and 100-200 μm wide. Typical exposures areabout 30 seconds in duration. By that method, Bragg gratings are readilyformed with, e.g., a constant period of about 0.5 μm.

The intensity profiles of the exemplary laser exposure sources describedabove are approximately Gaussian in shape. As a consequence, therefractive index perturbation in the resulting Bragg gratings appears asa periodic wave modulated by an envelope which is approximatelyGaussian. (And of course the perturbation, averaged over many periods,has essentially the same shape as the envelope.) This is an example ofan envelope which is at least partially pulse shaped. By "pulse shaped",we mean that as axial position is increased along some continuoussegment of the grating, the refractive index of the core rises to amaximum value and subsequently falls to a lower value. (Fine structurewould also be produced by a grating having a negatively pulse-shapedenvelope; i.e., an envelope that falls to a minimum value, instead of amaximum value, near the center. Although such gratings are atypical, weintend to include them under the definition of gratings withpulse-shaped envelopes.)

We have found a simple conceptual model that helps, in a qualitativefashion, to explain the resonant behavior of a pulse-shapedperturbation. This model is explained with reference to FIG. 3. Forcertain wavelengths of light near the central peak of the Bragg grating,the average refractive index (i.e., the index averaged over many, e.g.10, grating periods) in the grating is represented as three rectangularpulses having respective refractive indices n₁, n₂, and n₃, with n₁ ≈n₃,and n₁ <n₂ >n₃. The wavelength λ of light propagating within each pulseis related to the corresponding vacuum wavelength λ_(vac) by ##EQU1##where n is the relevant refractive index.

For wavelengths near the center of the main reflectivity peak of thegrating, the Bragg condition is satisfied in the central pan of theaverage refractive index profile (i.e., in the second pulse). However,some smaller wavelengths will satisfy the Bragg condition in the firstand third pulses, but fail the Bragg condition in the central pulse. Asa consequence, electromagnetic radiation of these wavelengths will bereflected in the first and third pulses. Within the second pulse, bycontrast, this radiation will be freely propagating and non-reflected,and it will be at least partially confined between the two reflectiveend pulses. This defines a Fabry-Perot cavity. The reflectivity spectrumof such a grating will exhibit fine structure generally lying atwavelengths below the center of the main reflectivity peak. The peaks inthis fine structure correspond to standing waves in the Fabry-Perotcavity.

According to a more detailed analysis, the Bragg grating has a designwavelength λ₀, which is the vacuum wavelength of peak reflectivity of anidealized grating having a given period and an average refractive indexn₀ equal to that of the unperturbed core material. As the exposure ofthe grating to the interfering beams is increased in intensity orduration, the perturbation is increased without changing the period ofthe grating. As a consequence, the peak vacuum wavelength is changedfrom λ₀ to a different wavelength, given by ##EQU2## Because increasingthe exposure generally increases the refractive index, this detuning isgenerally toward longer vacuum wavelengths. Because of the intensityprofiles of the interfering beams, this effect generally varies withaxial position within the grating.

An idealized grating is reflective with respect to λ₀ throughout theentire grating. (Art equivalent way to state this is that light at thewavelength λ₀ will have an evanescent component over the entire lengthof the grating.) Assume now that the grating is stronger in the centerthan at the ends, but there is no detuning. In that case, vacuumwavelengths slightly higher or lower than λ₀ will be evanescent only inthe central part of the grating, where the grating is strongest. Thissituation is depicted graphically in FIG. 4. The deviation of any givenvacuum wavelength from λ₀ is measured along the vertical axis, labeled Δin the figure. The axial coordinate is labeled z. Appearing as a shadedregion in the figure is the locus of all points (z,Δ) at which the Braggcondition is satisfied. To determine what portion of the grating isreflective with respect to a given vacuum wavelength, a horizontal lineis drawn at the corresponding value of Δ. The intersection of the shadedregion with the line is projected onto the z axis. This projectiondefines the axial extent of the reflective portion of the grating.

The effect of detuning is now discussed with reference to FIG. 5. Assumethat (as is usual) the detuning of the grating is toward longer vacuumwavelengths. Because the grating is now most efficient for somewavelength longer than λ₀, the detuning shifts the entire shaded regiontoward longer wavelengths and shifts the main reflectivity peak from λ₀to a longer vacuum wavelength λ₁. However, if the interfering beams havea pulse-shaped profile, the detuning is stronger in the center than atthe ends of the grating. This causes the central portion 20 of theshaded region to shift more than the end portions 25. As a consequence,there may be some range of wavelengths shorter than λ₁ that arereflected by the weaker, but less detuned, end portions of the grating,but are not reflected by the stronger, but more detuned, centralportion. With respect to these wavelengths, the central potion maybehave like a resonant cavity, in analogy to the central pulse of theconceptual model discussed above. In the same analogy, the reflectiveend portions 25, hereafter "wings", correspond to the two pulsesbounding the central pulse. This situation is depicted graphically inFIG. 5.

As noted, the detuning of any portion of the grating can be increased byincreasing the exposure of that portion to the index-modifyingradiation. As the detuning increases, the corresponding portion of theshaded area of FIG. 5 shifts toward longer wavelengths. It is apparentfrom the figure that a resonant cavity can be avoided by shifting atleast one of the wings 25 toward higher wavelengths, such that it willno longer reflect the light that otherwise resonates in the centralportion of the grating. This shift is readily produced by impinging abeam of actinic radiation, from a single, non-interfering beam, on thegrating. This index-modifying beam is displaced from the center of thegrating. The changes in the fine structure resulting from such treatmentare readily observed, and can even be monitored during the treatment.The treatment can be terminated when the fine, structure is reduced toan acceptable level.

An exemplary non-interfering, index-modifying exposure is illustrated inFIGS. 6-9. The unmodified grating is the grating that yielded (incomputational simulations) the spectrum of FIG. 1. This grating has adesign wavelength of 1557.45 nm, a peak index change, due to actinicexposure, of 0.08%, and a full width at half maximum of 1.75 nm. It isformed in a lossless optical fiber having a cladding refractive index of1.45. FIG. 6 shows the refractive index profile of the unmodifiedgrating. The refractive index is expressed in terms of the correspondingvacuum Bragg wavelength, as explained above. In the figure, curve 30represents the spatiaily averaged refractive index, and curves 35 and 40represent, respectively, the highest and lowest wavelengths that will bereflected in the grating.

The index-modifying exposure is described by FIG. 7. The actinic beamhas a Gaussian profile having the same spatial extent as the unmodifiedgrating. The center of the index-modifying beam is displaced to theposition where the profile of FIG. 6 falls to e⁻² times its peak value.The (simulated) beam profile of FIG. 7 is expressed as the the totalrefractive index profile (in terms of equivalent vacuum wavelength) thatthis exposure would produce in the pristine, photosensitive medium. FIG.8 is the simulated refractive index profile of same grating aftermodification by the exposure of FIG. 7. With reference to the regionbetween curves 35 and 40, it is apparent that the right-hand wing ofFIG. 6 is eliminated, and the left-hand wing is reduced to a very narrowregion that will contribute little or no fine structure.

FIG. 9 is the transmissivity spectrum of the modified grating. It isevident that the resolved subsidiary peaks 15 of FIG. 1 have beeneliminated.

The transmissivity spectrum of the grating of FIG. 9 shows anattenuation band. Attenuation, in its broadest sense, denotes radiationwhich is not transmitted through the waveguide. Such radiation may bereflected or escape through the waveguide. The attenuation band of FIG.9 has a full width at half maximum on the order of 1.5 nm (illustratedas "w" in the FIG.) and a peak attenuation at about 1558.5 nm. Becauseof the absence of fine structure, the attenuation exceeds 50% of thepeak attenuation everywhere within the full width at half maximum. Thatis, there are no subsidiary peaks having high transmission. Such agrating is said to be substantially "free of holes" since it does notsubstantially transmit radiation within the attenuation band.

The peak attenuation of the Bragg grating is greater than 90%.Preferably, the peak attenuation is greater than 95%, with peakattenuations of 98% and 99% being exemplary.

FIG. 13 illustrates a transmissivity spectrum for a linearly chirpedgrating formed with approximately 4.8 nm per cm linear chip rate. Anexemplary grating is at least partially formed within the core of anoptical waveguide but the invention is not limiting to gratings withinthe core. FIG. 14 is a schematic of the reflectivity spectrum for thegrating of FIG. 13. The spectrum includes a band in which incidentradiation is reflected back down the waveguide core. The reflectionwithin the band is greater than 90% over at least 2 contiguousnanometers with reflection greater than 95%, 90%, or 99% beingexemplary.

According to a further aspect of the present invention, a glass opticalwaveguide having a core with a Bragg grating at least partially formedwithin the core is formed. The Bragg grating has a transmission spectrumthat includes a band in which incident radiation is attenuated. Thetransmission within the band is less than 5% over at least 3 contiguousnanometers.

The invention also relates to a glass optical waveguide having a coreand a Bragg grating formed within the waveguide. The Bragg grating has atransmission spectrum that includes a band in which incident radiationis attenuated. The transmission within the band is less than 1% of thepeak transmission over at least 2 contiguous nanometers. An exemplarywaveguide is an optical fiber.

The index-modifying exposure is not limited to exposures having Gaussianprofiles. We believe that various other kinds of spatially modulatedexposure are also readily achieved, and in some cases may be desirable.For example, a ramp-shaped exposure can be achieved by scanning thegrating region with a relatively narrow actinic beam. During thescanning, the average intensity of the beam is varied, exemplarily as alinear function of axial position. The average intensity of a continuouslaser beam is readily modulated by, e.g., varying the input power to thelaser. The average intensity of a pulsed laser beam is readily modulatedby, e.g., varying the pulse repetition rate.

As noted, the fine structure typically appears at wavelengths below thecenter of the main reflectivity peak of the grating. In some cases, itmay be desirable to shift this structure to wavelengths above the centerof the main peak, rather than to eliminate it. With reference to thesimplified model of FIG. 3, the position of the fine structure can beinverted relative to the main peak by inverting the first pulse and thethird pulse relative to the second pulse. That is, a single,non-interfering beam can be used to selectively expose the regions ofthe first and third pulses such that the average refractive index ismade larger in these regions than in the central region. This result isshown in FIG. 10. As a consequence of this treatment, the vacuumwavelength that satisfies the Bragg condition will be greater for thefirst and third pulses than for the second pulse. Thus, there will besome wavelengths above the center of the main reflection peak that arereflected in the side pulses but not in the central pulse.

It should be noted that the index-modifying exposures discussed abovemay decrease the strength of the exposed portion of the grating. Thus,there will be a practical limit on the maximum index change that can beattained.

An alternative method for modifying the refractive index is to heat thegrating. We have observed that in at least some photosensitive glasses,heating of the Bragg grating tends to erase the grating; i.e., todecrease the amplitude of the refractive index perturbation.Accordingly, results similar to those described above can be achieved byemploying localized heating to modulate the average refractive index ofthe grating. Such localized heating is readily achieved by impinging abeam from an infrared laser, such as a carbon dioxide laser, on thephotosensitive medium. This beam is optionally displaced along themedium during the exposure. Various other methods of localized heatingare also possible, such as heating from the nearby tip of a heatedneedle-like probe.

A second way to eliminate, enhance, or invert the fine structure is tomodulate the grating period. This technique may be practiced instead of,or in addition to, the technique of modulating the average refractiveindex. According to this second technique, the vacuum wavelength thatsatisfies the Bragg condition in a given region is increased byincreasing the grating period in that region, and decreased bycorrespondingly decreasing the grating period. The fine structure can beeliminated by increasing the grating period on one side of the grating.This is achieved, for example, by forming a grating whose period varieslinearly with axial position (i.e., a linearly chirped grating). It iswell known that a chirped grating can be formed in photosensitivematerial by impinging thereupon a pair of interfering, non-collinear,converging, actinic beams. Because the beams converge, the effectiveintersection angle is dependent upon position. The local grating periodvaries according to variations of the intersection angle. The effect ofsuch a modulation on the spectral behavior of the grating will bequalitatively similar to the effect of adding a linearly varyingrefractive index perturbation. Indeed, it will be readily apparent tothe skilled practitioner that modulations of the grating period willproduce numerous effects analogous to those produced by modulating theaverage refractive index.

A preferred method for producing a chirped grating involves the use ofthe interferometer of FIG. 2, and is described in the co-pending U.S.patent application filed by V. Mizrahi et at. under the title "Methodfor Forming Spatially-Varying Distributed Bragg Reflectors in OpticalMedia." This application has issued as U.S. Pat. No. 5,363,239. Briefly,a tunable source of actinic radiation is used, and the actinicwavelength is varied during the exposure while, at the same time, theinterference pattern is displaced along the photosensitive medium.Alternatively, the actinic wavelength is held constant and theintersection angle of the actinic beams is varied. This is achieved bysubstituting a curved mirror for one of the fixed, planar mirrors of theinterferometric optical system, and displacing the interference patternduring the exposure.

In some cases, it may actually be desirable to enhance the finestructure, rather than to mitigate it. For example, it is possible toproduce a very sharp central peak in the grating reflectivity spectrumby superimposing fine structure that is very pronounced. This may beuseful in applications where an extremely sharp spectral peak isdesirable, and the presence of neighboring peaks can be tolerated. Thefine structure may be enhanced by further detuning a selected centralportion of the grating. Such further detuning can be performed byimpinging a single, actinic beam on the selected grating portion. Forexample, the grating may be exposed by a centered beam having a Gaussianprofile of the same spatial extent as the unmodified grating. Thesimulated transmissivity spectrum of the grating of FIG. 6 after such anexposure is shown in FIG. 11. As will be evident from the precedingdiscussion, similar results can be achieved by spatially modulating thegrating period.

In some cases, the fine structure of an unmodified grating will addroughness to the main peak by superimposing one or more well-resolvedsubsidiary peaks on the slowly varying background contributed by theside of the main peak. Such a situation is schematically depicted inFIG. 12, where subsidiary peak 45 (shown, fully resolved, as curve 50)adds roughness to the spectrum, whereas subsidiary peak 55 (shown, fullyresolved, as curve 60) merely broadens the main peak withoutsubstantially roughening it. When, e.g., a laser is locked onto thecentral peak of a Bragg grating by feedback stabilization, roughness isundesirable because it increases ambiguity about the peak location andincreases the likelihood that the tuning of the amplifier will jump tothe subsidiary peak. Accordingly, one application of the inventivemethod is to reduce the roughness added by the fine structure. One wayto quantify the roughness is to express it in terms of the height of thesubsidiary peak above the slowly varying background. This height isdesirably normalized to the height of the maim peak. Accordingly, theroughness corresponding to feature 45 of FIG. 12 may be expressed as theratio ##EQU3## For purposes such as tuning of an amplifier, the gratingis desirably modified such that the maximum roughness added by anysubsidiary peak is less than 10%.

We claim:
 1. Device for transmitting radiation comprising: a length ofglass optical waveguide having a core and an axial coordinate z, saidlength comprising a portion having a core refractive index perturbationδ(z) that varies as a function of z, wherein δ(z) is effective forproviding in said portion; a Bragg grating having a transmissionspectrum that includes a wavelength band in which incident radiation isattenuated, with peak attenuation being at a wavelength λ;saidwavelength band having a full width at half maximum, measured intransmission, that is greater than or equal to (1.5 nm/1558.5 nm) λ, andpeak attenuation within the wavelength band is greater than or equal to90%; wherein attenuation exceeds 50% of the peak attenuation everywherewithin the full width at half maximum.
 2. The device of claim 1 whereinsaid waveguide is an optical fiber.
 3. The device of claim 1 wherein thepeak attenuation is greater than 95%.
 4. The device of claim 1 whereinthe peak attenuation is greater than 98%.
 5. The device of claim 1wherein the peak attenuation is greater than 99%.
 6. Device fortransmitting radiation comprising:a length of glass optical waveguidehaving a core and an axial coordinate z, said length comprising aportion having a core refractive index perturbation δ(z) that varies asa function of z, wherein δ(z) is effective for providing in said portiona Bragg grating having a reflectivity spectrum that includes awavelength band in which incident radiation is reflected; whereinreflection within said wavelength band is greater than 90% over at least2 contiguous nanometers of wavelength.
 7. Device for transmittingradiation comprising:a length of glass optical waveguide having a coreand an axial coordinate z, said length comprising a portion having acore refractive index perturbation δ(z) that varies as a function of z,wherein δ(z) is effective for providing in said portion a Bragg gratinghaving a transmission spectrum that includes a wavelength band in whichincident radiation is attenuated; wherein the transmission within saidwavelength band is less than 5% over at least 3 contiguous nanometers inwavelength.
 8. The device of claim 7 where the transmission within saidwavelength band is less than 5% over at least 5 contiguous namometers.9. Device for transmitting radiation comprising: a length of glassoptical waveguide having a core and an axial coordinate z, said lengthcomprising a portion having a core refractive index perturbation δ(z)that varies as a function of z, wherein δ(z) is effective for providingin said portion a Bragg grating having a transmission spectrum thatincludes a wavelength band in which incident radiation is attenuated,wherein the transmission within said wavelength band is less than 1% ofa peak transmission over at least 2 contiguous nanometers in wavelength.10. Device of claim 9 wherein said waveguide is an optical fiber.
 11. Aspectrally selective optical waveguide, made by the method comprising:a)providing a photosensitive, optically waveguiding glass body having atleast one waveguiding axis; and b) forming in said body a firstrefractive index perturbation that defines a Bragg condition relative tolight propagation parallel to the waveguiding axis, and that exhibits areflectivity spectrum having a main peak relative to vacuum wavelengthsof light that approximately satisfy the Bragg condition, wherein theforming step comprises the further steps of: c) generating aninterference pattern of actinic radiation having a spatially averagedintensity profile that includes at least one rising portion and at leastone falling portion; and d) impinging the interference pattern on atleast a portion of said body such that the interference pattern has atleast a locally defined period along the waveguiding axis, and saidintensity profile rises and falls along the waveguiding axis, theimpinging step performed so as to lead to formation of the firstrefractive index perturbation; wherein: e) the impinging step leads to aspatially-averaged, refractive-index profile in the first refractiveindex perturbation that has at least one rising portion and at least onefalling portion along the waveguiding axis, and at least one segment, tobe referred to as a resonant segment, included between the rising andfalling portions, whereby, absent any further exposure to actinicradiation, the reflectivity spectrum comprises at least one subsidiarypeak,CHARACTERIZED IN THAT the method further comprises: f) producing asecond refractive index perturbation in the body, resulting in areduction of the amplitude of the subsidiary peak relative to the mainpeak, such that said reflectivity spectrum comprises a wavelength bandwherein reflection is greater than 90% over at least 2 contiguousnanometers of wavelength.
 12. A spectrally selective optical waveguidehaving a reflectivity spectrum, made by the method comprising:a)providing a photosensitive, optically waveguiding glass body having atleast one waveguiding axis; and b) forming in said body a firstrefractive index perturbation that defines a Bragg condition relative tolight propagation parallel to the waveguiding axis, and that exhibits areflectivity spectrum having a main peak relative to vacuum wavelengthsof light that approximately satisfy the Bragg condition, wherein theforming step comprises the further steps of: c) generating aninterference pattern of actinic radiation having a spatially averagedintensity profile that includes at least one rising portion and at leastone falling portion; and d) impinging the interference pattern on atleast a portion of said body such that the interference pattern has atleast a locally defined period along the waveguiding axis, and saidintensity profile rises and falls along the waveguiding axis, theimpinging step performed so as to lead to formation of the firstrefractive index perturbation; wherein: e) the impinging step leads to aspatially-averaged, refractive-index profile in the first refractiveindex perturbation that has at least one rising portion and at least onefalling portion along the waveguiding axis, and at least one segment, tobe referred to as a resonant segment, included between the rising andfalling portions;CHARACTERIZED IN THAT the method further comprises: f)during (d), causing the period to vary approximately linearly along thewaveguiding axis, such that the resonant segment will not supportsubstantial Fabry-Perot resonances, such that said reflectivity spectrumcomprises a wavelength band wherein reflection is greater than 90% overat least 2 contiguous nanometers of wavelength.